The present invention generally relates to optical sensing systems for sensing gases or particulates entrained in a fluid, and more specifically, to a method for developing a detailed spatial data map of a specific gas concentration or particulate concentration within a fluid by using a relatively simple optical beam geometry.
A variety of instruments developed for the optical remote sensing of atmospheric contaminants produce measurements that integrate the contaminant concentration over the length of the optical beam path. Such measurements are commonly referred to as Path Integrated Optical Remote Sensing (PI-ORS) measurements, and have characteristic units of concentration times length, e.g. ppm-meters.
The beam paths used in such measurements can range from a few meters to several kilometers in length. Often, the measurements are interpreted by dividing the path-integrated measurement of contaminant concentration by the path length to obtain the average concentration over the path. While providing useful information, the average concentration data are often difficult to interpret, as it is generally preferably to know the contaminant concentration at specific locations over the measurement path, rather than just the average concentration for the entire path length. The prior art teaches that by using complex beam geometries with a variety of existing PI-ORS instrumentation, and Computed Tomography (CT), the path integrated concentration (PIC) measurement data can be converted into two-dimensional (2D) spatial maps to facilitate air pollution mapping that indicate the location of specific sources of air contamination in the area region being surveyed.
In order to apply conventional CT mathematical techniques to this type of measurement, the optical beam paths must be arranged so that multiple beams from a variety of projection viewpoints intersect or overlap. This requirement necessitates the use of multiple instruments and multiple detectors, or the use of complicated schemes for scanning and folding the beam paths with mirrors to achieve a dense overlapping beam geometry. The concept of combining PI-ORS and CT for mapping outdoor air contaminant concentrations was introduced in theoretical studies during the late 1970s. These studies were based on systems that included complex beam geometries requiring multiple mirrors and detectors. The optical systems proposed were difficult to align and maintain and were very costly. Furthermore, for path lengths much greater than a few tens of meters, alignment difficulties could render such complex beam-based systems essentially unworkable.
In view of the problems with such prior art optical systems, it clearly would be desirable to develop a method for obtaining and manipulating PI-ORS data to produce spatial maps without the need for a complex beam arrangement to provide PIC data. Preferably, such a method should rely on a far simpler beam geometry, using radial beam paths of varying length, projected outward over the sampling area from a single source. Such a simple beam geometry, in combination with various conventional reconstruction algorithms, or an algorithm specifically designed for a radial non-overlapping beam geometry, should facilitate more rapid data collection , and should be usable both for indoor and outdoor applications. A simple radial beam geometry also could be applied to retrieve one-dimensional (ID) reconstructions of the concentration profile along a fence line, street, of other linear region of interest. The prior art does not teach or suggest such a method.
In accord with the present invention, a method is defined for mapping contaminants within a sampling region using path-integrated data. The method includes the steps of providing an instrument capable of generating path integrated concentration (PIC) data within the sampling region, and using the instrument to acquire PIC data for a plurality of different paths from within and extending to the boundaries of the sampling region. A cumulative distribution function that fits the acquired PIC data is reiteratively generated. Using the cumulative distribution function, a map of the contaminants within the sampling region is created.
Although a preferred form of the invention employs a plurality of light beams to scan the sampling region, the instrument can produce an illuminating signal that is either an optical signal or an acoustic signal. The step of using the instrument to acquire PIC data comprises the steps of using the instrument to generate a first path having a first length, and then using the instrument to generate additional paths having different lengths within the sampling region. It is expected that the instrument will be used to generate at least three paths of different lengths for a 1D reconstruction. In one embodiment, the instrument used to acquire PIC data generates a plurality of non-intersecting paths. In addition, the plurality of paths are preferably arrayed about a substantially common origin.
An embodiment of the instrument includes an illuminating unit and a detector, and a reflective unit adapted to reflect a light signal emitted from the illuminating unit to the detector. In one embodiment, a reflective unit is provided for each of the plurality of paths and the reflective units are adapted to direct a signal emitted from the illuminating unit back to the detector. In this case, an orientation of the instrument varied to achieve different directions for each of the plurality of different paths.
The cumulative distribution function which is fitted to the observed PIC data can be specified by a matrix of discrete pixel values, a matrix of spline coefficients, as a continuous function, or as a set of overlapping smooth basis functions. The step of reiteratively generating the cumulative distribution function preferably continues so long as a level of improvement in the fit of the cumulative distribution function to the acquired PIC data between successive iterations exceeds a signal noise level associated with the instrument, or some other objective stopping criteria.
If known parameters for the contaminant distribution are used with said cumulative distribution function, fewer paths can be required to acquire the PIC data. For example, the known parameters may include the locations of contaminant source(s) within the sampling region or a concentration range of the contaminants within the sampling region.
Another aspect of the present invention is directed at apparatus for acquiring PIC data from within a sampling region and generating a spatial concentration map based on the PIC data. The apparatus includes an instrument for emitting energy along a path, at least one detector, a memory for storing machine instructions, and a processor coupled to the detector and the memory, for executing the machine instructions to carry out functions that are generally consistent with the method described above.